One type of non-volatile memory, known as magnetic random access memory (MRAM), includes an array of magnetic memory cells. The magnetic memory cells may be of different types, such as magnetic tunnel junction (MTJ) memory cells or giant magnetoresistive (GMR) memory cells. Typically, a magnetic memory cell includes a layer of magnetic film in which the orientation of magnetization is alterable and a layer of magnetic film in which the orientation of magnetization may be fixed or “pinned” in a particular direction. The magnetic film having alterable magnetization is referred to as a sense layer or data storage layer and the magnetic film that is fixed is referred to as a reference layer or pinned layer.
Conductive traces referred to as word lines and bit lines are routed across the array of memory cells. The word lines extend along rows of the memory cells and the bit lines extend along columns of the memory cells. A memory cell stores a bit of information as an orientation of magnetization in the sense layer at each intersection of a word line and a bit line. The orientation of magnetization in the sense layer aligns along an axis of the sense layer referred to as its “easy axis”. The orientation of magnetization does not easily align along an axis orthogonal to the easy axis, referred to as the “hard axis”. Magnetic fields are applied to flip the orientation of magnetization in the sense layer along its easy axis to either a parallel or anti-parallel orientation with respect to the orientation of magnetization in the reference layer. The resistance through the memory cell differs according to the parallel or anti-parallel orientation of magnetization and is highest when the orientation is anti-parallel, i.e. one logic state, and lowest when the orientation is parallel, i.e. the other logic state.
In one configuration, a write circuit is electrically coupled to the word lines and the bit lines to write the state of a memory cell. The write circuit selects one word line and one bit line to change the orientation of magnetization in the sense layer of the memory cell situated at the conductors crossing point. A write current is passed through a word line to create a magnetic field along the hard axis and another write current is passed through a bit line to create a magnetic field along the easy axis. The hard axis magnetic field loosens the sense layer orientation of magnetization and the easy axis magnetic field flips the sense layer orientation of magnetization along the easy axis to switch the state of the memory cell. The magnitudes of the magnetic fields in the selected memory cell surpass levels needed to set or switch the state of the memory cell. The margin by which the magnitudes surpass the levels needed is referred to as a write margin. A large write margin ensures that the selected memory cell is written. However, an easy axis magnetic field alone can change the state of a memory cell.
The non-selected memory cells along the selected word line and bit line are presented with only one magnetic field. These memory cells are referred to as half-selected memory cells. The margin between the magnitude of the magnetic field in the half-selected memory cell and the level needed to switch the half-selected memory cell is referred to as the half-select margin. A large half-select margin ensures that half-selected memory cells will not be inadvertently switched.
The magnitudes of the magnetic fields needed to switch the state of a memory cell vary from cell to cell across the array. Some selected memory cells will not switch if the write currents and subsequent magnetic fields are too small. Alternatively, some half-selected memory cells will switch if the easy axis magnetic field surpasses a certain magnitude. Intermittent and inadvertent switching problems call for extra error correction mechanisms or the array is gradually rendered unreadable. Increasing the write margin and half-select margin reduces these problems and makes for a more reliable magnetic memory.